The objective of this work is to determine the importance of the human cytochrome P450 2D6 (CYP2D6) drug metabolizing polymorphism in drug abuse and dependence. About 10% of the Caucasian population lack CYP2D6 (poor metabolizer or PM phenotype), the enzyme responsible for metabolizing several drugs of abuse (e.g. codeine, oxycodone, paramethoxyamphetamine [PMA], dextromethorphan) to their active metabolites. In the rest of the population (extensive metabolizers [EMs]) the enzyme shows great variation in its activity and it can be potently inhibited by many drugs (e.g. fluoxetine, MMDA, methadone). The role of CYP2D6 genotype and phenotype as a risk or protective factor for drug abuse or toxicity will depend on whether a specific drug is active or has active metabolites and in EMs on absolute activity of the enzyme. In pre-clinical studies we plan to: 1) Identify and characterize more drugs of abuse, particularly relating to the phenethylamines (e.g. methamphetamine [MAP] and PMA) that are substrates or inhibitors of CYP2D6 using human liver microsomes and CYP2D6 expressed in yeast and CNS-derived cell lines; 2) Define the localization, catalytic properties and regulation of CYP2D forms in rat, monkey and human brain using PCR amplification, restriction digests and functional assays with 3- methoxymorphinan as substrate; 3) Determine if brain CYP2D is important in the conversion of the trace amine tryptamine to serotonin ex vivo and in vivo and if this is important in the regulation of drug self- administration; 4) Using rats, model the importance of CYP2D deficiency and high catalytic activity to the toxicity and behavioral consequences of amphetamines, as models of substrates that are drugs of abuse with less active metabolites. In four human studies we will determine: 1) The frequency of and correlation with CYP2D6 phenotype and genotype for MAP, d-amphetamine, dextromethorphan and in opiate non-responders; 2) The efficacy of inhibiting CYP2D6 with fluoxetine compared to placebo and sertraline in a 24 week double blind treatment efficacy trial in 150 patients dependent on prescription opiates (e.g. codeine, oxycodone); 3) Dextromethorphan abuse liability in 18 EM and 12 PM recreational drug users compared to ketamine, a phencyclidine analog, and in a multiple choice drug discrimination test to ketamine and triazolam; 4) The metabolism, kinetics and abuse liability of MAP and AMP in 12 EM and 6 PM recreational drug users. We expect these studies will: identify some drugs of abuse for which the EM or PM state or PM phenocopying can modify the risk of abuse; explain why some drugs of abuse are so attractive or toxic to some individuals and why some patterns of drug abuse are endemic (e.g. PMA, dextromethorphan, smoked methamphetamine, "designer drugs"); determine the role and importance of the localized brain CYP2D catalytic activity; result in a treatment of prescription opiate dependence, the third largest drug problem in North America. Our studies may contribute to psychopharmacological approaches to the treatment of mental disorders, and to understanding brain function involving endogenous neurotransmitters, neurosteroids and neuroactive steroids.